1. Field of Invention
The present invention relates to a method for forming self-aligned contact window. More particularly, the present invention relates to a method for forming a self-aligned contact window that allows a self-aligned silicide process and a self-aligned contact process to be run on the same device.
2. Description of Related Art
In today's ultra large-scale integrated circuit fabrication, devices have now shrunk to sub-quarter micron dimensions. As device dimensions continue to shrink, the application of self-aligned silicide material and the forming of self-aligned contact (SAC) window are methods for saving device areas. Since a contact window opening is formed on the surface of a gate region in a self-aligned contact process, the top surface of the gate region must be covered by an insulating layer to prevent any electrical contact with the contact window. Because an insulating layer such as silicon nitride is difficult to form over a silicide layer other than a tungsten silicide layer, the gate must be covered by a tungsten silicide layer thereby limiting the use of a conventional self-aligned contact process.
FIGS. 1A through 1C are cross-sectional views showing the progression of manufacturing steps in forming a conventional self-aligned contact window. First, as shown in FIG. 1A, a substrate 10 is provided. Then, a polysilicon layer 11, a tungsten silicide layer 12 and a silicon nitride layer 13 are sequentially formed over the substrate 10. The purpose of the silicon nitride layer 13 is for preventing the tungsten silicide layer 12 from making contact with other conductive layers. Thereafter, the silicon nitride layer 13, the tungsten silicide layer 12 and the polysilicon layer 11 are patterned to define a gate region 14. The gate region 14 comprises of the polysilicon layer 11 and the tungsten silicide layer 12. Next, using the silicon nitride 13 and the gate region 14 as a mask, ions are implanted into the substrate 10 forming lightly doped drain (LDD) regions.
Thereafter, as shown in FIG. 1B, spacers 16 are formed on the sidewalls of the gate region 14 and the silicon nitride layer 13. The spacers 16 are formed, for example, by first forming a silicon nitride layer over the substrate surface 10, and then performing an anisotropic etching back operation. Next, using the spacers 16 and the silicon nitride layer 13 as a mask, ions are again implanted into the substrate 10 to form the source/drain regions 17.
Next, as shown in FIG. 1C, a dielectric layer such as a silicon dioxide layer is formed over the substrate 10 using, for example, a chemical vapor deposition method. Thereafter, the dielectric layer 18 is patterned to define a contact window 19 exposing the source/drain region 17, the spacer 16 and a portion of the silicon nitride layer 13. Due to the presence of a silicon nitride layer 13 over the gate region 14, when the dielectric layer 18 is etched to form the contact window 19, the silicon nitride layer 13 can act as an etching stop layer. Hence, electrical contact between the contact window 19 and the gate region 14 is prevented.
The aforementioned conventional method of forming a self-aligned contact is only suitable for gate region having a tungsten silicide on top. The reason for this is that the quality of a silicon nitride deposition is good only when the underlying layer is tungsten silicide. The deposition of a silicon nitride layer over a titanium silicide layer is unsatisfactory. Consequently, when a self-aligned titanium silicide layer has already formed over the device, it will be very difficult to form a silicon nitride layer that acts as an etching stop layer over the titanium silicide layer by a conventional method. Therefore, using a self-aligned silicide process to form a self-aligned titanium layer is incompatible with using a conventional self-aligned contact process to form a self-aligned contact on the same device for reducing device occupational area.
In light of the foregoing, there is a need to provide an improved method for forming a self-aligned contact.